1) Field of the Invention
The present invention relates to a gain control apparatus, optical transmission apparatus and gain control method for optical amplifier, and more particularly to a technique suitable for use in a WDM (Wavelength Division Multiplex) optical transmission system for achieving long-distance transmission by using optical fiber amplifiers, represented by the Erbium-doped fiber amplifiers (EDFAs), in a multi-stage fashion.
2) Description of the Related Art
In recent years, as one example of a WDM optical transmission system, attention has been attracted with respect to a metrocore system which makes connections among local based cities and which is capable of carrying out the add/drop of optical signals with arbitrary wavelengths at arbitrary nodes. FIG. 7 is a block diagram showing an example of a configuration of a metrocore system. In this system shown in FIG. 7, OADM nodes 100, which are a plurality of optical transmission apparatus, are connected through transmission lines (optical fibers) 110 into a ring-like configuration so that, at each of the OADM nodes 110, a signal light with an arbitrary wavelength (channel) can be added to the transmission line 110 and, of WDM signal light propagating along the transmission lines 110, a signal light with an arbitrary wavelength can be dropped therefrom.
In addition, each of the OADM nodes 100 is composed of an OADM unit 101 which carries out the add/drop of a signal light, a front-end amplification unit 102a serving as a pre-amplifier located at a former and a latter stage of the OADM unit 101, and a back-end amplification unit 102b serving as a post-amplifier (when discrimination is not made between these amplification units 102a and 102b, they will be referred to as optical amplification units 102). The amplification operations of these optical amplification units 102 enable compensating for the loss of the signal light level between the OADM nodes (sometimes, each of which will hereinafter be referred to simply as a “node”) 100 adjoining to each other, thereby lengthening the transmission distance.
Still additionally, in the case of the above-mentioned system enabling the add/drop of a signal light at an arbitrary node 100, the number of signal wavelengths (or the number of transmission wavelengths) propagating in the system (transmission lines 110) varies dynamically and, for coping with this variation of the number of wavelengths, an AGC amplifier 120 having an automatic gain control (AGC) function is usually employed for the above-mentioned optical amplification units 102 in order to maintain a constant output light power with respect to each wavelength (keep gain flatness for each wavelength).
In this case, for example, as shown in FIG. 8, the AGC amplifier 120 is composed of an EDFA 121, optical branching units 122 and 123 such as optical couplers for respectively splitting partially input/output lights to/from the EDFA 121, PDs 124 and 125 for respectively monitoring the powers of the branch lights (i.e., input/output light powers of the EDFA 121) split by the optical couplers 122 and 123, and an automatic gain control unit 126 for controlling the excitation light power to the EDFA 121 so that the power ratio monitored by the PDs 124 and 125 becomes constant. The SOFA 121 includes an EDE (not shown) serving as an amplification medium, an excitation light source, and an optical coupling unit for supplying the excitation light of the excitation light source from the former-stage or latter-stage of the EDF.
Meanwhile, in the system shown in FIG. 7, let it be assumed that, for example, as shown in FIG. 9(A), a large number of (for example, 39 wavelengths) optical signals are added from a certain node 100 (100A) and a different one-wavelength optical signal is added from the next node 100 (100B). In such a situation, for example, as shown in FIG. 9(B), in a case in which a dynamic re-construction of an optical transmission path or a trouble such as a man-made mistake, fiber disconnection or fallout of a connector occurs between the nodes 100A and 100B, only the signal light added at the node 100B remains (that is, the number of transmission wavelengths varies abruptly). At this time, for example, as shown in FIG. 10(A), there occurs a phenomenon of a variation of the power level of the residual optical signal at a signal reception end under the node 100 (100C).
In this case, for example, as shown in FIG. 16, the aforesaid “signal reception end” signifies an optical receiver 131 having an optical/electrical conversion (O/E) function to receive a dropped light split at the node 100 (100C in FIG. 9(A)) for converting it into an electric signal, and this also applies to the following description. Moreover, a “signal transmission end” signifies an optical transmitter 132 having an electrical/optical conversion function (E/O) to convert a transmission signal (electric signal) into an optical signal. An optical signal from the optical transmitter 132 is inserted as an added light into a WDM signal light at nodes 100 (100A and 100B in FIG. 8(A)).
For example, as shown in FIG. 10(B), the aforesaid signal light power variation stems mainly from three factors: (1) spectral hole burning (SHB), (2) gain (wavelength) deviation and (3) Stimulated Raman Scattering (SRS) effect. Each of these factors will be described hereinbelow.
(1) SHB
The first factor “SHB” is a physical phenomenon occurring at an optical amplification in an optical amplifier such as the EDFA 121 and shows a feature that the shorter wavelength side optical signal power falls. That is, for example, as shown in FIG. 11, when an optical signal with one wavelength (for example, 1538 nm) in the C band (1530 to 1565 nm) is amplified in the EDFA 121, the possible phenomenon is that the EDFA gain in the vicinity of the signal wavelength drops (which is referred to as main hole) and the EDFA gain in the vicinity of 1530 nm also falls (which is referred to as second hole).
In addition, in the C band, there is a characteristic that the main hole becomes deeper toward the shorter wavelength side (the gain falling degree increases), and the main hole and the second hole become deeper as the optical signal input power increases. This SHB is subjected to the averaging in a state where a multi-wavelength signal light is inputted thereto and the influence thereof is low, whereas the influence thereof increases as the number of inputted wavelengths decreases.
For this reason, for example, as shown in the row (1) of FIG. 10(B) and as shown in FIG. 12(A), in a case in which a trouble has occurred between the nodes 100A and 100B as mentioned above, if a signal light with a wavelength inserted (added) from the node 100A drops out and only a signal light with a wavelength inserted at the node 100B remains (see the time t0 in FIG. 10(A)), there occurs a phenomenon that the gain of the EDFA 121 serving as the back-end optical amplification unit 102b in the node 100B further decreases (−ΔP) in the case of the shorter wavelength side residual signal light, which also causes a reduction of the output light power.
In the example shown in the row (1) of FIG. 10(B), one wavelength λa on the shorter wavelength side (inserted or added at the node 100B) and a longer wavelength band L remain while, when an intermediate wavelength band M positioned on the longer wavelength side with respect to λa (inserted at the node 100A) drops out, the output light power of the shorter wavelength side signal light with the residual wavelength λa decreases by ΔP in comparison with the output light power in the longer wavelength side wavelength band L.
Thus, in a case in which a variation of a waveband amplified by the optical amplification unit 102 occurs, as well known, the influence of the SHB applied to the residual signal after the variation varies according to the number of wavelengths of the residual signal light and the location or arrangement thereof. That is, the fluctuation degree of the gain in the optical amplification unit 102 due to the SHB varies. The detailed description about the SHB exists in detail in the Non-Patent Documents 1 to 3.
(2) Gain Deviation
The second factor “gain (wavelength) deviation” is a phenomenon occurring the optical amplification units 102 (102a, 102b) configured as the AGC amplifier 120. That is, as mentioned above, the optical amplification units 102 are designed to execute control (AGC) for maintaining a constant average gain of signal light and, when a wavelength producing a deviation remains, the optical amplification unit 102 operates so as to adjust the gain of the signal light to a target gain and, for example, as shown in the row (2) of FIG. 10(B), with respect to the residual optical signal, the output light power from the optical amplification unit 102 varies (in this case, +ΔP). That is, in this case, the operating point in the optical amplification unit 102 varies according to the number of signal wavelengths and the location thereof, which causes a variation of the gain spectrum.
Even in a case in which a signal light for each wavelength, which has originally a uniform level, is inputted before the occurrence of a trouble between the nodes 100A and 100B, in the back-end optical amplification unit 102b serving as the AGC amplifier 120 in the node 100B, a WOM signal light having a power distribution shown on the left side in the row (2) is outputted from the EDFA 121 (see FIG. 8) due to a gain wavelength characteristic of the EDFA 121. The example shown in the row (2) of FIG. 10(B) shows a case in which, of the wavelength components of the WOM signal light amplified by the EDFA 121, a wavelength λb having an output power lower than the output power which is an object of the AGC control before the occurrence of a trouble remains at the occurrence of the trouble.
That is, at the execution of the AGC control, when the full wavelength band B (inserted at the node 100A) other than λb drops out due to the trouble occurring between the nodes 100A and 100B while one wavelength λb of the intermediate wavelengths (inserted at the node 100B) remains, in the back-end amplification unit 102b of the node 100B, the target gain (operating point) is increased in the automatic gain control unit 126, and the light power after the amplification of the signal light with the remaining wavelength band λb reaches an light power which is a target of AGC control. Therefore, the signal light with this wavelength λb is increased by ΔP in comparison with the value before the variation of the wavelength location.
(3) SRS Effect
The third factor “SRS effect” is a phenomenon occurring in the transmission lines 110 (see FIG. 7). The Raman amplifier is an optical amplifier utilizing this SRS effect. For example, as shown in FIG. 13, the SRS of a common single mode fiber shows a feature that a gain peak appears on a lower frequency side shifted by approximately 13 THz from the excitation light wavelength (longer wavelength side by approximately 100 nm in a case in which the excitation light wavelength is in the vicinity of 1400 nm), and the optical signal amplification in an arbitrary wavelength band becomes feasible by the selection of an excitation light wavelength. However, as shown in FIG. 13, difficulty is experienced in enabling the amplification of a pinpoint wavelength, and the amplification (gain) characteristic has some degree of spread with respect to wavelength and, hence, the amplification phenomenon occurs even in the vicinity of the excitation light wavelength.
That is, in a case in which a WDM optical signal is transmitted long the transmission lines 110, the shorter wavelength side signal light power becomes the excitation light power, which amplifies the longer wavelength side signal light. In consequence, as shown in FIG. 14, the phenomenon appears that the signal light power increases toward the longer wavelength side. Accordingly, in a case in which only one-wavelength signal light remains due to the occurrence of a trouble between the nodes 100A and 100B as mentioned above, as shown in the row (3) of FIG. 10 and in FIG. 12(B), the longer wavelength side residual signal light encounters a larger difficulty to take the power from the shorter wavelength side, and a power (gain) reduction (−ΔP) occurs. That is, the SRS effect varies according to the number of signal wavelengths and the location thereof.
Thus, if the number of wavelengths of a WDM signal light propagating in the transmission lines 110 varies largely, due to the influence of the SHB, gain deviation and SRS described above, the output light power of the residual signal light (residual channel) varies. In the OADM unit 101, it is possible to place a function to adjust the levels of the signal lights with the respective wavelengths through the use of the feedback control, and this feedback control function can cope with the fluctuation of the output light power stemming from the aforesaid variation of the number of signal light wavelengths and the locations.
However, this respective-wavelengths handling feedback control usually takes a considerable long time from the variation of the number of wavelengths (time t0) up to the steady-state functioning (time t2) as shown in FIG. 10(A). That is, difficulty is actually encountered in suppressing even the transient fluctuation of the output light power during the time t1 in FIG. 10(A) after the variation of the number of wavelengths.
In addition, with respect to such a transient fluctuation of the output light level, although the fluctuation for each node 100 or for each transmission line 110 is not very large, the power fluctuation characteristic of one optical amplification unit 102 increases cumulatively in the case of the system in which similar optical amplification units 102 carrying out the AGC are provided in a multi-stage fashion. For example, as shown in FIG. 15, the negative-side output light power variation (−ΔP) for each channel which occurs at each optical amplification unit 102 and transmission line 110 increases cumulatively due to an increase in number of spans, i.e., an increase in number of stages of the transit optical amplifiers. The example in FIG. 15 shows that the negative-side output power variation increases due to an increase in number of spans.
In the case of a conventional optical transmission system which provides a short transmission distance and which has a small number of stages of the optical amplifiers, this variation is minute and does not create a problem. However, in the future, along with a further increase in the number of stages of optical amplifiers for the long-distance transmission of the system, as shown in FIG. 15, the optical signal power at a signal reception end exceeds a reception allowable range, which can trigger a transmission error. In other words, in a wavelength multiplex optical transmission system which performs a repeating transmission through a plurality of nodes 100 each having an OADM function, there is a possibility that the occurrence of a trouble in one transmission line zone can affect the communication in another transmission line zone.
Furthermore, as techniques related to the invention of the present application, there are the techniques disclosed in the following Patent Documents 1 to 4.
The Patent Documents 1 and 2 disclose that a portion of signal light inputted to an optical amplifier is taken out as monitor light and inputted to a wavelength demultiplexer (DEMUX) for demultiplexing it according to wavelength for counting the number of transmission wavelengths. Concretely, the technique disclosed in the Patent Document 1 is designed to monitor inputted light to an optical amplifier for each wavelength for adjusting the attenuation quantity in a variable optical attenuator provided at an output of the optical amplifier according to the monitor value and a variation of the number of wavelengths, thus controlling the output light power collectively. On the other hand, according to the technique disclosed in the Patent Document 2, in an optical amplifier where optical amplification fibers such as EDFs are connected in a multi-stage fashion, the excitation light power to each optical amplification fiber and the attenuation quantity in a variable optical attenuator provided between the stages of the respective optical amplification fibers are adjusted on the basis of a signal light power detected from input light to the former-stage optical amplification fiber, the number of wavelengths and a signal light power detected from output light of the latter-stage optical amplification fiber, thereby controlling the gain and gain spectrum of the entire optical amplifier.
As techniques for compensating (flattening) for the aforesaid variation (deviation in wavelength characteristic of signal light) of the output from an optical amplifier, there are a dynamic gain equalizer (DGEQ) and the techniques proposed in the following Patent Documents 3 and 4. The technique disclosed in the Patent Document 3 is designed to use an optical circulator, an optical reflector, a variable optical attenuator and a WDM coupler for carrying out the gain equalization for each of a plurality of signal lights (wavelengths) split by the WDM coupler. The technique disclosed in the Patent Document 4 relates to a variable gain flattening device including a plurality of gratings having a long-period structure and an adjustment unit (a piezo converter and a piezo control circuit) for adjusting the attenuation rate for each grating.
[Patent Document 1] Japanese Patent Laid-Open No. 2001-168841
[Patent Document 2] Japanese Patent Laid-Open No. 2003-258348
[Patent Document 3] Japanese Patent Laid-Open No. HEI 10-173597
[Patent Document 4] Japanese Patent Laid-Open No. HEI 11-337750
[Non-Patent Document 1] Masato NISHIHARA, et. al., “Characterization and new numerical model of spectral hole burning in broadband erbium-doped fiber amplifier”, 2003 Optical Society of America
[Non-Patent Document 2] Masato NISHIHARA, et. al., “Impact of spectral hole burning in multi-channel amplification of EDFA”, 2004 Optical Society of America
[Non-Patent Document 3] Maxim Bolshtyansky, “Spectral Hole Burning in Erbium-Doped Fiber Amplifiers”, JOURNAL OF LIGHT WAVE TECHNOLOGY, VOL. 21, NO. 4 APRIL 2003.
However, in the case of the above-mentioned level adjustment function in the OADM unit 101 and the technique disclosed in the Patent Document 3, since the received WDM signal light is demultiplexed according to wavelength and the optical power is adjusted for each wavelength by variable optical attenuator for each wavelength, the apparatus scale becomes larger and the cost becomes higher. In particular, when a VOA which can achieve a high-speed operation is used for obtaining a high-speed response characteristic, this VOA is costly and, if the VOAs equal in number to the wavelengths are prepared, the cost further increases. If a relatively low-cost and low-speed operating VOA is employed from the viewpoint of cost restriction, it is impossible to suppress the above-mentioned transient output light power fluctuation.
In addition, the above-mentioned dynamic gain equalizer creates a problem, for example, in that the response speed stands at approximately 30 ms, the cost is high and reaches several millions yen and the insertion loss is large (approximately 5 dB) and, hence, difficulty is experienced in actually introducing it into the system. The technique disclosed in the aforesaid Patent Document 4 individually changes the characteristics of a plurality of gratings by controlling a piezo converter through the use of a piezo control circuit for changing the pressure to be applied to the grating, thereby enabling a variation of the attenuation rate of light passing through the grating. However, since the pressure change, i.e., physical control, is conducted with respect to the grating, the response speed becomes low as well as the above-mentioned technique.
The techniques disclosed in the Patent Documents 1 and 2 are not made to suppress the transient variation of the gain wavelength characteristic of the EDFA 121 stemming from the above-mentioned variation of the number of wavelengths and then restrain the transient fluctuation of the output light level.